a) Field of the Invention
This invention relates to an imaging system having a focus detecting device used for photographic cameras, cinecameras, video cameras, etc.
b) Description of the Prior Art
In the past, what is called a TTL system, which is free of parallax, has been often adopted as a focus detecting device for single-lens reflex cameras. In most cases, to facilitate the calculations of the amounts of a front focus and a rear focus in particular, the focus detecting device of a so-called phase correlation scheme is used in which photoelectric conversion means (light-receiving elements) are subjected to the intensity distributions of light of two images formed by two beams of light passing along two different optical paths or passing through different regions in the same optical system and focus detection is performed by detecting the phase difference between output signals indicative of the intensity distributions which are derived from the photoelectric conversion means.
An example of the conventional focus detecting device of this type is schematically shown in FIG. 1 with reference to the optical system of the device which is incorporated in the single-lens reflex camera and commercialized. This optical system comprises a photographic lens 1; a swing-up type instant-return mirror 3 composed of a half mirror, pivotally mounted in the mirror box 2; a sub-mirror 4 pivotally mounted on the back surface of the instant-return mirror 3, located perpendicular to the instant-return mirror 3 when the instant-return mirror 3 is set at the lower position (shown) and coming in close contact with the back surface of the instant-return mirror 3 when the instant-return mirror 3 is shifted to the upper position; an imaging plane 5 (a film plane in the case of a camera for photography with silver halide) arranged, behind the sub-mirror 4, on the rear face of the mirror box 2; a field stop 6 arranged, on the optical path of reflection from the sub-mirror 4, at a plane equivalent optically to the imaging plane 5; a condenser lens 7 arranged adjacent to the field stop 6; a mirror 8 arranged behind the condenser lens 7; an aperture stop 9 arranged behind the mirror 8, having a pair of apertures juxtaposed normal to the plane of the figure at an interval which makes it possible to secure focusing accuracy; a pair of separator lenses (reimaging lenses) 10 juxtaposed, normal to the plane of the figure, behind the aperture stop 9 having the pair of apertures, respectively; a light-receiving element 11 arranged at the imaging position brought about by the separator lenses 10; a focusing screen 12 arranged, on the optical path of reflection from the instant-return mirror 3, at a plane equivalent optically to the imaging plane 5; a condenser lens 13 disposed in front of the focusing screen 12; a field stop 14 disposed at the same position as the focusing screen 12; a pentagonal roof prism 15 whose entrance surface is positioned in rear of the focusing screen 12; and an eyepiece 16 arranged behind the exit surface of the pentagonal roof prism 15. Of these components, the photographic lens 1 and imaging plane 5 constitute a photographing optical system. Further, the instant-return mirror 3, submirror 4, field stop 6, condenser lens 7, mirror 8, aperture stop 9 with a pair of apertures, separator lenses 10 in a pair, and light-receiving element 11 constitute a focus detecting optical system on a phase correlation scheme of performing the focus detection by making use of beams of light traversing the different areas of the photographic lens 1 to detect the relative positional relationship of a pair of images of an object. Also, the instant-return mirror 3, condenser lens 13, focusing screen 12, field stop 14, pentagonal roof prism 15, and eyepiece 16 constitute a finder optical system.
Specifically, when the instant-return mirror is set at the upper position, a primary image I.sub.0 of an object point O is formed on the imaging plane 5 by the photographic lens 1. On the other hand, when the instant-return mirror 3 is set at the lower position, part of the imaging beams of light through the photographic lens 1 is transmitted by the instant-return mirror 3 and conducted to the focus detecting optical system, while the remainder, reflected from the instant-return mirror 3, is conducted to the finder optical system.
For the focus detecting optical system, the light beam transmitted through the instant-return mirror 3 is reflected at the sub-mirror 4 so that a primary image I.sub.1 of the object point O is formed on the field stop 6 and then, through the condenser lens 7, mirror 8, aperture stop 9 with a pair of apertures, and separator lenses 10 in a pair, a pair of secondary images I.sub.2 and I.sub.3 is formed on the light-receiving element 11.
As for the finder optical system, the light beam reflected from the instant-return mirror 3 is nearly collected by the condenser lens 13 so that a primary image I.sub.4 of the object point 0, after being formed on the focusing screen 12, is converted into an orthographic image, together with the image of the field stop 14, by the pentagonal roof prism 15, the orthographic image being magnified through the eyepiece 16 for observation at an eyepoint EP.
In the focus detecting optical system, the image of an exit pupil P at the photographic lens 1 is formed on the aperture stop 9 in virtue of the condenser lens 7. By the pair of apertures of the aperture stop 9 and the pair of separator lenses 10, two beams of light traversing the different areas of the exit pupil P are conducted onto the light-receiving element 11 while holding a certain base length (the distance between the centers of the light beams).
The foregoing prior art, however, has encountered the problems that the arrangement is complicated and the volume occupied by the focus detecting optical system increases, with the result that the entire camera body becomes bulky.
Further, the imaging system having the focus detecting optical system includes the arrangement that a beam of light for focus detection is separated from the imaging optical system while the optical path directed toward the imaging plane is not blocked.
This arrangement is such that focusing can be performed during the photography and sharp picture images are brought about even when the photography is sequentially carried out. An example in which such an arrangement is realized is set forth in Japanese Patent Preliminary Publication No. Sho 57-168205 and shown in FIG. 2. Reference numeral 23 denotes a focusing prism disposed between the imaging lens system 1 and the imaging plane 5, having a half mirror 24 which is a light-splitting surface. In such an optical system, a convergent beam of light leaving the imaging lens 1 enters the prism 23 from an entrance surface 23a arranged nearly perpendicualr to the optical axis so that part of the light beam is imaged at the imaging plane 5 and the remaining beam, after reflection from the half mirror 24, is reflected in sequence by the surfaces 23a and 23b of the prism 23, emerges from an exit surface configured at the same plane as the entrance surface 23a in a direction normal practically to the exit surface to travel nearly parallel to the optical axis of the imaging lens 1 as a beam of light utilized for focus detection, and forms an image of the object at a position nearly conjugate with the imaging plane 5. If, therefore, the photoelectric conversion device 11 is arranged at such an imaging position, the intensity distribution of the object image can be converted into an electrical signal for the focus detection. The preceding arrangement makes it possible to perform focusing during the photography and eliminate the effects of the irregularity of illuminance and the unevenness of spectral properties on the entire image surface because the half mirror 24 performs its function over the whole of an effective imaging light beam. Additionally, since the entire section of an axial light beam can be utilized as the light beam for detection, the focusing characteristic in the imaging plane 5 can be made equivalent to that in the detecting system, with the resultant improvement of the focusing accuracy.
This prior art, however, has the problems that the prism 23 splitting the light beam to be directed toward the photographic system and the focus detecting system is comprised of two prism elements and in addition, the coating of the half mirror 24 is applied to their cemented surface, so that the prism 23 is high in cost and complicated in structure.